Radiation source and method for making same



Dec.

15, 4 R. E. BLACK ETAL RADIATION SOURCE AND METHOD FOR MAKING SAME FiledMarch 31, 1960 INVENTOR.

BY {$5172 .reen

United States Patent Ofilice 3,161,504 Patented Dec. 15, 1964 3,161,504RADIATION SOURCE AND METHGD FOR MAKING SAME Robert E. Black, Utica, andFarno L. Green, Bloomfield Hills, Mich, assignors to General MotorsCorporation,

Detroit, ,Mich., a corporation of Delaware Filed Mar. 31, 1960, Ser. No.19,009 2 Claims. (Cl. 75-206) This invention relates to a photonradiation source and method for making same. It has as one of itsobjects the provision of an improved radiation source pellet wherein arare earth radioisotope serves as the photon emitter. Another object isthe provision of a photon source pellet which has improved structuralstrength and which therefore will withstand mechanical handling withoutany substantial loss of radioactive material. Another object is theprovision of aradioactive source pellet which comprises a mixture of aphoton-emitting rare earth and a nonradioactive metal which providesoptimum photon emission from the quantity of rare earth included.

In its broadest scope, the radioactive source pellet of this inventionis a dense sintered mixture of a rare earth radioisotope, which servesas the photon source material, and nonradioactive aluminum or columbium,preferably the former, which serves as the binder. The pellet is made byforming a mixture of aluminum or columbium powder and a rare earth orrare earth compound, preferably the oxide, pressing the mixture into apowder compact of the shape desired and then sintering the compact. Therare earth can be irradiated with thermal neutrons to itsphoton-emitting radioisotopic form either prior to forming the mixtureor by irradiating the pellet after the sintering operation. The rareearths samarium, gadolinium, europium, ytterbiurn and thulium areexcellent for the practice of the invention. In the preferredembodiments the concentration of the rare earth is greater in the centerportions of the pellet than in surface portions. In this manner thesurface portions of the pellet, which are rich in binder metal andtherefore of high ductility and elasticity, provide the pellet withoptimum structural strength, durability and resistance to loss ofradioactive material from mechanical handling.

The following detailed description of a preferred embodiment willprovide a clearer understanding of the invention, reference being madeto the appended drawings in which:

FIGURE 1 shows a sectional view of a cylindrical radioactive pellet madein accordance with the invention; and

FIGURES 2 and 3 illustrate the preferred method for making the pelletshown in FIGURE 1.

Referring now to FIGURE 1, the darkened area 1 in the center portion ofthe pellet indicates a relatively high concentration of Samarium-145oxide and the light portions 2 at the surface of the pellet indicate arelatively low concentration of said oxide and a high concentration ofaluminum. It is especially desirable that the portions immediatelyadjacent the top and bottom surfaces of the pellet be substantially freeof rare earth oxide such that the edges 3 will have optimum resistanceto cracking or crumbling from mechanical handling and such that thepellet can be contacted on its top and bottom surfaces without rub-offof radioactive material.

The following example, described with reference to FIGURES 2 and 3 ofthe drawings, will illustrate the preferred method for manufacture ofthe pellet.

First, the interior surfaces of a cylindrical die opening 4 and thefacing surfaces of mating punches 5 and 6 are lightly coated with atacky organic binder such as beeswax which can be applied in admixturewith a suitable solvent such as benzene. A charge of -200 to 325 meshaluminum powder is applied to the coated die such that a uniform layersticks to all the die surfaces, and the coated punches are in a likemanner covered with a layer of the aluminum powder, all as shown inFIGURE 2. Then the lower punch is inserted and the die opening filledwith a uniform mixture of powdered rare earth oxide in an amount up toabout by weight, and the remainder aluminum powder of the aforesaid meshsize. After the die is filled the upper punch, coated with aluminumpowder, is inserted and pressure on the order of 350,000 pounds persquare inch is applied to compress the said mixture into aself-sustaining compact as shown in FIG- URE 3. This compact is thenremoved from the die and heated to about 1000 F. for about one-half hourin air to cause sintering of the aluminum. The unitary pellet resultingfrom the sintering operation may then be further densified and shaped,if desired, by placing it in a suitable die opening and pressing it witha punch.

In some instances, particularly where the mixture used to form the mainbody of the pellet has a high percentage of aluminum, it may beadvantageous that only certain of the surface portions of the pellet beentirely of aluminum. For example, in the instance of a cylindricalpellet, the cylindrical side walls may, if desired, be of the samecomposition as the center portion. To make such a pellet a layer ofaluminum powder is placed at the bottom of the die cavity, then a layerof a mixture of rare earth oxide and aluminum powder, and over this atop layer of aluminum powder. Pressure is applied, and thethree-layeredcompact can then be sintered and further compacted asdescribed above. Such a pellet will have top and bottom edge portions(equivalent to those indicated at 3 in the embodiment shown in thedrawings) which are substantially entirely of aluminum and which aretherefore highly resistant to chipping and cracking. Also, theradioisotope-free top and bottom surfaces allow for handling of thepellet without rub-oif of radioactive material.

If it is desired to incorporate a greater amount of radioactive materialin the pellet, the surface portions instead of being formed entirely ofaluminum can be formed from an aluminum-rich mixture of aluminum andrare earth oxide. For example, the surface portions can be made of .amixture of 75 aluminum and 25% rare earth oxide and the center portionof 75% rare earth oxide and 25 aluminum.

After the pellet has been completely formed the rare earth within thepellet can be rendered radioactive by irradiation of the pellet in anuclear reactor. For example, a number of the pellets were successfullyirradiated by subjecting them to a flux of about 2X10 thermalneutrons/cmF/sec. for six weeks in the LITR at Oak Ridge, Tennessee.

The nuclear reactions of interest for typical rare earths during thethermal neutron irradiation are:

and the photon-emitting nuclear reactions of the radioiso topes soformed are:

Sm Pm-145 +photons Sm-153 Eu-153-|photons Eul55 Gd-155 +photons Gd-153Eu153+photons Tm- Er-170+pho tons Yb-169 Tm-169+photons it If desired,the rare earth can be irradiated prior to forming the pellet throughthis has the disadvantage of requiring the handling of radioactivematerial during pellet manufacture.

The radioactive pellets of this invention are useful as gamma radiation,X-ray and bremsstrahlung sources for radiography and for liquid levelgauges, density gauges and the like which are well known and which arecoming into common use in industry. They have the advantage of excellentmechanical strength and related physical properties such that there isgood assurance against loss of radioactive material in handling. Thephotons emitted are within relatively narrow energy levels and hence thesource pellets of this invention approach the ideal of a single energylevel emission. The columbium of aluminum, in combination with the rareearth, provides superior physical properties but has no significantadverse nuclear effect. Also, where the rare earth is irradiated afterpellet formation, the binder, separating as it does the rare earthparticles, diminishes the neuton shielding eifect of one rare earthparticle on another and thereby provides optimum radioactivation. Thepreferred embodiment wherein the rare earth concentration is low insurface portions of the pellet is particularly advantageous in thisrespect as well as with respect to mechanical strength.

It will be understood that while the invention has been describedparticularly with reference to preferred embodiments thereof, it is notso limited since changes and modifications may be made all within thefull and intended scope of the claims which follow.

We claim:

1. A method for making a photon radiation source comprising the steps ofcoating the surfaces of a die opening with a tacky organic material,applying to the tacky surfaces a powdered metal such that a layer of thepowdered metal is formed on said surfaces, said metal being selectedfrom the group consisting of aluminum, columbtium and aluminum-columbiumalloys, placing in the remaining portions of the die opening a powderedmixture consisting essentially of a rare earth material and said metal,adding a powdered layer of said metal over said mixture and thenpressing to form a composite powder compact having surface portions ofsaid metal and a center portion of said mixture and heating said compactto cause sintering thereof.

2. A method for making a photon radiation source comprising the steps ofcoating the surfaces of a die opening and mating upper and lower puncheswith a tacky organic material, inserting the coated bottom punch intothe die opening, applying to the tacky surfaces a powdered metal suchthat a layer of the powdered metal is formed on said surfaces, suchmetal being selected from the group consisting of aluminum, columbiumand aluminumcolumbium alloys, filling the coated die opening with apowdered mixture consisting essentially of a rare earth oxide and saidmetal, inserting the coated upper punch in the die opening and pressingto form a composite powder compact having surface portions of said metaland a center portion of said mixture, heating said compact to causesintering thereof, pressing the sintered body to increase its density,and then subjecting the sintered body to thermal neutron irradiation toconvert the rare earth material to a photon-emitting radioisotope.

References Cited in the file of this patent UNITED STATES PATENTS2,592,115 Carroll Apr. 8, 1952 2,805,473 Handwerk Sept. 10, 19572,814,849 Hamilton Dec. 3, 1957 2,866,741 Hausner Dec. 30, 19582,975,113 Gordon M Mar. 14, 1961

1. A METHOD FOR MAKING A PHOTON RADIATION SOURCE COMPRISING THE STEPS OFCOATING THE SURFACES OF A DIE OPENING WITH A TACKY ORGANIC MATERIAL,APPLYING TO THE TACKY SURFACES A POWDERED METAL SUCH THAT A LAYER OF THEPOWDERED METAL IS FORMED ON SAID SURFACES, SAID METAL BEING SELECTEDFROM THE GROUP CONSITING OF ALUMINUM, COLUMBIUM AND ALUMINUM-COLUMBIUMALOYS, PLACING IN THE REMAINING PORTIONS OF THE DIE OPENING A POWDEREDMIXTURE CONSISTING ESSENTAILLY OF A RARE EARTH MATERIAL AND SAID METAL,ADDING A POWDERED LAYER OF SAID METAL OVER SAID MIXTURE AND THEN PASSINGTO FORM A COMPOSITE POWDER COMPACT HAVING SURFACE PORTIONS OF SAID METALAND A CENTER PORTION OF SAID MIXTURE AND HEATING SAID COMPACT TO CAUSESINTERING THEREOF.